Earlier today, Cydia started saving SHSH blobs of iOS 4.2.1 on new-generation devices only since I couldn't see the SHSH blobs on my iPhone 3G but it's been saved on iPhone 4. This happens because you'll get the untethered jailbreak after few hours !
For those who don't know, the iPhone Dev Team will release untethered Redsn0w 0.9.6b7 jailbreak for iOS 4.2.1 or what's known by the backup plan today. We'll of course be posting this as soon as it's available for download and will be back with full how-to guides quickly. Stay tuned !